During boring by a boring turbine, the cutting forces produce an axial thrust which cannot be controlled because it depends largely on the operators' manipulations and the nature of the ground. The weight of the turbine rotor and, if applicable, an additional hydraulic axial thrust are absorbed by the turbine bearing. The axial thrust due to the cutting forces is directed in the opposite direction to the thrust due to the weight of the turbine rotor. However, since the axial thrust due to the cutting forces is considerably greater, these forces must be absorbed by several roller or ball bearings, which are arranged adjacent to each other. Several small roller or ball bearings are needed since the space is restricted in the radial direction and does not allow for a single bearing of appropriately large dimensions.
Intermediate spring rings have been arranged between the roller or ball bearings to distribute the load over these bearings. Despite the theoretical uniformity of the working life of these bearings, individual bearings have failed prematurely. The working life of the bearing block was thus reduced considerably, which had an adverse effect on the already high cost of boring.
Uneven loading of the bearings has occurred for the following reason. Intermediate spring rings are normally operated over a characteristic deflection curve range in which the force on the rings also increases with the length of the path of the axial motion of the turbine shaft. This force on each ring is retransmitted to the bearing associated with the intermediate ring. However, different bearing play prevails in the bearings, within their tolerance limits. Thus, before the intermediate spring rings respond to the occurrence of an axial thrust on the shaft of the boring tool, and before they are loaded further when there is corresponding initial stressing, this bearing play in the roller bearings, which is hereafter referred to as the idle path, must be taken up. The idle path is indeterminate within the tolerance limits of the bearing. This results in different loadings for the individual bearings. The intermediate spring rings are normally used during the rising portion or range of the characteristic curve of the rings, after the idle path has been traversed. In the extreme case, when the idle path of a bearing is greater than the total length of the path of axial motion of the boring shaft due to the axial thrust on the shaft, it can even happen that that bearing does not take any load at all. This leads to corresponding overloading of the other bearings.